Past studies by Dr. Mark Bertness have shown that crabs and mussels by themselves can have positive effects on plant growth – most likely because crabs can reduce the stress of low oxygen in the sediments by building their burrows, and mussels can add nutrients to the sediments.
Figure 3 from Mark Bertness's 1984 Ecology study illustrating the positive effects of mussel presence (white bars) on Spartina
Table 3 from Mark Bertness's 1985 Ecology study. Fiddler removal has a negative effect on Spartina in the marsh flat, but not the marsh edge.
Although both fiddlers and mussels occur together in the field, no studies have looked at how the combination affects the plants. Are the positive effects of each species by itself doubled? Or are they redundant with each other? Do crabs somehow reduce the positive effect of mussels, or vice-versa? How many crabs or mussels do you need to get a positive effect on Spartina? These are some of the questions that we hope to answer with our experiment.
Our new deck at FSUCML.
But first, we had to get everything set up. There were several long and hot days of shoveling sand into our “mesocosms” (10 gallon buckets) – many thanks to Robyn, Chris, Althea, and all the others who took care of that task! Then there was another day spent transplanting the Spartina.
Chris, Randall, and Robyn work to transplant Spartina from the greenhouse to the mesocosms.
Finally, it was time to add the fiddlers and mussels, and everything began!
Mussels nestled among the Spartina stems in one of our experimental mesocosms
Althea and Chris have been leading the charge on this experiment, and they’ve spent a lot of time getting to know (and identify) the fiddler crabs. All in all, a pretty fun study organism!
Althea working to identify fiddler crab species.
We’ll continue the experiment another month and then measure the height and density of the plants in each treatment to see if there are any differences. Once this experiment is complete, we’ll set up a separate one asking somewhat of the converse question – are two enemies (periwinkle snails and grasshoppers) worse than one? We’ll keep you posted.
Randall’s research is funded by the National Science Foundation.
Randall examines an experiment cage as Robyn looks on.
Calling a one year experiment an “era” is probably a bit of an over-statement, but the end of our snail field experiment definitely feels significant. Especially for Robyn, who has traveled to St. Joe Bay at least once a week for the past year to count snails and take other data. And also for the Webbs, who were kind enough to let us put cages up in the marsh right in front of their house and then proceed to show up to check on them at odd hours for the last year! And finally for this blog, because the beginning of the snail experiment was the first thing we documented last summer when we started this project with WFSU. It’s nice to come full circle.
So why, you may wonder, are we ending things now? Is it simply because one year is a nice round number? Not really, though there is some satisfaction in that. The actual reasons include:
(1) The experiment has now run long enough that if snails were going to have an effect on cordgrass, we should have seen it by now. (At least based on prior studies with these same species in GA.)
(2) In fact, we have seen an effect of periwinkle snails, and in some cages there are very few plants left alive for us to count! (And lots of zeros are generally not good when it comes to data analysis.)
(3) Perhaps the most important reason to end things now: it’s become increasingly difficult in some cages to differentiate the cordgrass that we transplanted from the cordgrass that is growing there naturally. Being able to tell them apart is critical in order for our data to be accurate.
(4) The results of the experiment have been consistent over the last several months, which increases my confidence that they are “real” and not simply some fluke of timing or season.
And what are the results? As I mentioned above, snails can have a really dramatic effect on cordgrass, most noticeably when our experimental transplant is the only game in town (i.e., all the neighboring plants have been removed). And not surprisingly, cordgrass does just fine in the absence of snails and neighbors – they’re not competing with anyone or being eaten!
Snails also have a pretty strong effect on the experimental cordgrass transplant (compared to when no snails are present) when all of its neighbors are cordgrass.
Most interestingly, snails do not have a big effect on the experimental cordgrass transplant when some of the neighboring plants are needlerush.
This result is consistent with some of the patterns we’ve observed in natural marshes, where cordgrass growing with needlerush neighbors is taller and looks “healthier” than nearby cordgrass growing without needlerush.
Having decimated the plants in the cage, the snails move towards the tallest structure they can reach- a PVC pipe.
But why? Those snails are pretty smart. They generally prefer to climb on the tallest plant around, because it gives them a better refuge at high tide when their predators move into the marsh. (We’ve shown this refuge effect in the lab – fewer snails get eaten by blue crabs in tanks with some tall plants than in tanks with all short plants.) Needlerush is almost always taller than cordgrass in the marshes around here, so this preference for tall plants means that snails spend less time on cordgrass when needlerush is around. And finally, less time on cordgrass means less time grazing on cordgrass, so the cordgrass growing with needlerush experiences less grazing pressure.
These results – consumer (snail) effects on cordgrass are lower when cordgrass grows mixed with needlerush – are consistent with theory on the effect of diversity, even though in this case we’re only talking about a “diversity” of 2 plant species. And they could be important in the recovery or restoration of marsh areas where snails are causing a large reduction in cordgrass biomass.
The one thing we still don’t know with certainty – how do the snails determine which plant is taller??
I guess that’s the beauty of this job, in that there are always more questions to answer.
Randall’s research is funded by the National Science Foundation.
The new documentary, In the Grass, On the Reef: Testing the Ecology of Fear had a segment on the snail experiment. Watch the full program here. You can also read Randall’s post from the beginning of the experiment, and watch a video, here.
Emily holds a net that will soon be full of marsh bugs. Later, at the lab, she will identify the many insect species that live amongst fiddlers and periwinkles, species more often associated with the habitat.
Several weeks ago, I went to Houston to meet Thomas Decker, a tech in Steve Pennings’ lab at the University of Houston. Thomas graciously offered up his time to help me with my insect identifications. I have absolutely zero background in entomology, the study of bugs and other creepy crawlies most people squirm about. So how did I end up spending hours puzzling through an identification book on insects, a book with so many unfamiliar terms that I was constantly flipping to the glossary and various diagrams? Silly me, I decided that I needed to include the terrestrial part of my salt marsh community. Which meant I have spent quite a bit of one-on-one time with a dichotomous key on insects. A dichotomous key is a “choose your own adventure” style guide to identification. Continue reading →
If you’re a regular reader of the blog, you’ll realize that we often talk about similar research questions or ideas in the context of different projects. As David mentioned in his description of the Baymouth Bar project, this overlap is usually intentional: as ecologists, we’re interested not only in the specific habitats that we study, but also in the underlying factors that affect these habitats and the valuable services that they provide to we humans.
It may appear at times that we’ve been covering a diverse array of topics, and while this is true, all of these topics are interconnected- a web of topics centered around a couple of central themes. The diagram below is the map that shows where every post-topic fits into these central themes. Even the artists, writers, and photographers we occasionally feature have their place amongst ecological processes like sedimentation and the non-consumptive effects of predators. Every post from here on out will have one of these icons on it- if you don’t know what the icon means, just click on it and you’ll be back at this figure with an explanation:
Katie LotterhosFSU Department of Biological Sciences, FSU
When we look at a salt marsh, we see thousands of stems of cordgrass. But in reality, the coastline may be made up of only a few different genetic individuals. This is because Spartina can spread by growing clones of itself, with the exact same genetic code (a genotype). Why does it matter if we know whether or not a salt marsh is made up of one or many different genotypes? Well, different genotypes will have different abilities to resist pests or disease, or they may be tastier to eat for the little marsh critters like snails and grasshoppers. Since some genotypes will be better than others in different situations, we care about genetic diversity because it can be a buffer against an uncertain environment.